A "perfect storm" of a fire in northern NSW has destroyed 28 homes, with firefighters expecting that number to rise.

The fire has burned through 40,000 hectares near the Warrumbungle National Park.

After Sunday's destructive rampage it remained uncontained on Monday, with a 100km front, although no further properties were immediately under threat.

But Rural Fire Service (RFS) Deputy Commissioner Rob Rogers told reporters that the number of homes destroyed was expected to go up as crews inspected burnt-through areas.

He said 40 farm sheds had also been lost along with many livestock, much fencing and farm machinery.

"There was just absolutely no stopping that fire," Mr Rogers said.

He said the wind shifted to the south at the worst possible time, creating "perfect storm" conditions for a fire that burnt "with such a ferocity we haven't seen in years".

Mr Rogers said evacuations were absolutely necessary and police had enforced that.

A number of buildings, including the lodge at the Siding Spring Observatory in the national park, were damaged in the bushfire that firefighters say was fanned by "atrocious' weather conditions.

But Andrew Hopkins, acting director of the Australian Astronomical Observatory, said the Anglo-Australian Telescope was believed to have survived the blaze.

"What we understand at this point in time is that there has been some damage to some of the structures on the top of the mountain but it looks as though all of the main telescope facilities are okay," Hopkins said.

"Of course we're incredibly grateful to the efforts of the Rural Fire Service. The significance of the site is absolutely huge. The Siding Spring Observatory site is the premiere astronomical observatory facility in Australia for optical and infrared observing."

Eighteen staff members were evacuated from the observatory, which is owned by the Australian National University.

About 100 residents were evacuated last night along a four-kilometre fire front, with people seeking refuge in Coonabarabran as huge columns of smoke filled the sky.

"It looked like an atom bomb the way it went up," said Susan Armstrong, who owns a property to the west of the fire. Her husband, Brian, had been fighting the blaze, which started on Saturday, but his crew had to pull back.

"They got sent home. It was far too dangerous," Armstrong said. "He said the spot fires and how quickly it all moved was quite scary."

FIRE CREW DEATH

In Tasmania, a 60-year-old firefighter from Victoria was found dead near a hamlet on the Tasman Peninsula where he was conducting a backburn.

Police said the man was part of a Victorian contingent assisting the Tasmania Fire Service.

He was preparing for backburning operations at Waterfall Bluff, which is about two to three kilometres from the active fire edge.

Workers were sent to the area after the man failed to make a scheduled call-in.

The latest blast of heat from Australia's hot heart sent the mercury soaring over the weekend in parts of western NSW and in Queensland. Walgett touched 48.5 degrees and nearby Bourke 48. Tibooburra Post Office's 47.9 degrees was the highest in 103 years of records.

''Once we get to Tuesday to Thursday of [this] week, the interior could be just as hot as today,'' Brett Dutschke, senior meteorologist at Weatherzone, said.